The present invention relates generally to communications systems, more specifically, to cellular radio communications base stations and associated methods.
Cellular communications systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Analog cellular systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
Frequency reuse is commonly employed in cellular technology wherein groups of frequencies are allocated for use in regions of limited geographic coverage known as cells. Cells containing equivalent groups of frequencies are geographically separated to allow mobile units in different cells to simultaneously use the same frequency without interfering with each other. By so doing many thousands of subscribers may be served by a system of only several hundred frequencies. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands called channels. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 MHz. At present there are 832, 30-KHz wide, radio channels allocated to cellular mobile communications in the United States. To address the capacity limitations of this analog system a digital transmission standard has been provided, designated IS-54B, wherein these frequency channels are further subdivided into 3 time slots.
As illustrated in FIG. 1, a cellular communication system 20 as in the prior art includes one or more mobile stations or units 21, one or more base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of base stations, thousands of mobile stations and more than one MTSO. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. A typical cell may have, for example, one control channel, and 21 voice/data, or traffic, channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information.
The MTSO 25 is the central coordinating element of the overall cellular network 20. It typically includes a cellular processor 28, a cellular switch 29 and also provides the interface to the public switched telephone network (PSTN) 30. Through the cellular network 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or, between a mobile station 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication with the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals. The base station 23 also supervises the quality of the link 32 and monitors the received signal strength from the mobile station 21.
A typical base station 23 as in the prior art is schematically illustrated in FIG. 2 which shows, as an example, the functional components of model number RBS 882 manufactured by Ericsson Telecom AB, Stockholm, Sweden for the CMS 8800 cellular mobile telephone system. A full description of this analog cellular network is provided in publication number EN/LZT 101 908 R2B, published by Ericsson Telecom AB.
A now common sight along many highways, the base station 23 includes a control unit 34 and an antenna tower 35. The control unit 34 comprises the base station electronics and is usually positioned within a ruggedized enclosure at, or near, the base of the tower. Within this enclosure are the radio control group 37, or RCG, an exchange radio interface (ERI) 38 and a primary power supply 41 for converting electric power from the AC grid to power the individual components within the base station 23, and a backup power supply 42.
The ERI 38 provides signals between the MTSO 25 and the base station 23. The ERI 38 receives data from the RCG 37 and transfers it to the MTSO 25 on a dedicated MTSO-BS link 45. In the reverse direction, the ERI 38 receives data from the MTSO 25 and sends it the RCG 37 for subsequent transmission to a mobile station 21.
The radio control group 37 includes the electronic equipment necessary to effect radio communications. A functional block diagram of an RCG 37 as in the prior art is shown in FIG. 3. The configuration shown illustrates one control channel transmit/receive module (TRM) 51, a number of voice channel TRMs 52, and one signal strength receiver 53, as is a typical configuration required to serve one cell or sector of a cell. Each TRM 51, 52 includes a respective transmitter 54, receiver 55 and control unit 57. The TRMs 51, 52 typically are not frequency agile and operate instead on only one predetermined channel. Control signals from the ERI 38 are received by the individual control units 57. Voice and data traffic signals are routed over a separate interface to the ERI 38.
Each individual transmitter 54 for control and voice is connected to a transmit combiner 58. The transmit combiner combines all of the input signals onto a single output coupled through a coaxial cable 62 to the transmit antenna 63. Through the use of the combiner 58, up to 16 transmitters 54 can typically be connected to a common transmit antenna 63. The combiner 58 is used because there is often a premium for space on the masts and towers used to support the antennas. In an extreme case, one mast may be required to support over 100 radio channels.
One visible feature of a typical base station 23 is the antenna tower 35. In order to achieve a reasonable coverage area, the antennas 63, 65 are desirably mounted at some distance above the ground. Referring now additionally to the prior art schematic plan view illustration of FIG. 4A, in rural areas the towers 35 are commonly located at the center of a cell 36 thereby providing omni-directional coverage. In an omni-directional cell, the control channel(s) and the active voice channel(s) are broadcast in all areas of the cellxe2x80x94usually from a single antenna. Where base stations 23 are more densely located, a sectorized antenna system may be employed as in the prior art, and shown by the schematic diagram of FIG. 4B. Sectorization requires directional antennas 70 having, for example, a 120 degree radiation pattern as illustrated in FIG. 4B. Each sector 71 is itself a cell having its own control channel(s) and traffic channel(s). Note that xe2x80x9cchannelxe2x80x9d may refer to a specific carrier frequency in an analog system or to a specific carrier/slot combination in a hybrid TDMA/FDMA system, such as IS-54 and GSM.
Each of two receive antennas 65 is coupled to a respective receive combiner 66A, 66B where the signals received are separated according to frequency and passed on to the individual receivers 55 in each of the TRMs 51, 52. The signals received often suffer from the detrimental effects of interference and fading. For this reason the two receive antennas 65 are typically spaced a distance apart, often more than ten times the carrier signal wavelength, so that they may receive signals from different signal paths which exhibit uncorrelated fading, thereby providing spatial diversity reception.
Diversity reception involves combining signals from two or more antennas. There are many conventional techniques for both pre-detection and post-detection diversity combining which are described, for example, in Chapter 10 of the book entitled xe2x80x9cMobile Communications Engineeringxe2x80x9d, by William C. Y. Lee, published by McGraw-Hill, 1992. A typical diversity combiner takes a signal from each of two antennas, assigns a weighting factor to each signal according to indices of quality, such as carrier-to-noise ratio or mean signal power, and combines the signals according to these weighting factors to produce a single enhanced quality signal.
Despite the theoretical advantages of space diversity in cellular system base station receiving antennas, sufficient separation may not always be obtainable. In other words, the antenna tower may not permit a sufficient physical separation of receive antennas to achieve uncorrelated fading for receive signals. This also applies in applications using beamforming techniques, which often require that elements of the receive antenna array be separated by a wavelength or less. In addition, the orientation of the linearly polarized mobile antenna may not always be in alignment with the typically vertically polarized receive antenna at the base station, causing polarization mismatch fading.
It is known that polarization diversity reception may be used to enhance signal quality. Polarization diversity reception takes advantage of the low correlation between signals of differing polarizations. It involves combining signals of differing polarizations, typically orthogonal to each other, using diversity combining techniques similar to those employed in spatial diversity techniques. Polarization diversity reception is described, for example, in Chapter 9 of xe2x80x9cMobile Communications Engineeringxe2x80x9d, by William C. Y. Lee.
For both spatial and horizontal diversity reception, it is possible to achieve greater diversity gain by adding additional antennas and passing the additional signals received into a higher order diversity combiner. However, this approach increases computational and hardware complexity due to the need for a more complex higher-order diversity combiner. As many existing base stations employ two spatially diverse antennas coupled to a two-branch diversity combiner, adapting these stations for use with multiple antennas may necessitate replacement of existing two-branch combiners. Moreover, the increase in gain achieved with higher-order diversity combining generally diminishes with increasing order, and additional noise is injected from signal branches exhibiting poorer quality, such as low carrier-to-noise ratio or high levels of interference.
In the light of the foregoing, it is therefore an object of the present invention to provide cellular radiotelephone base stations capable of enhanced communication with cellular radiotelephones, particularly in view of fading and/or interference.
It is another object of the present invention to provide improved cellular radiotelephone base stations and methods for enhanced cellular radiotelephone communications without the need to add undue complexity to the design of cellular radiotelephone base station elements.
These and other objects, advantages, and features of the present invention are provided by a cellular radiotelephone base station which produces at least three processed radiotelephone communications signals from a cellular radiotelephone antenna array and signal selecting means for selecting at least two but less than all of the processed radiotelephone communications signals. The selected signals may then be passed to a decoder, such as a conventional two-branch combiner. The base station can thereby be adapted for use with multiple antennas, without requiring undue replacement of other hardware elements.
In particular, according to the present invention, a cellular radiotelephone base station includes a cellular radiotelephone receive antenna array for receiving cellular radiotelephone communications energy representing cellular radiotelephone communications from cellular radiotelephones. Cellular radiotelephone communications processing means is responsive to the antenna array, for processing the received cellular radiotelephone communications energy to produce at least three processed radiotelephone communications signals, each representing the cellular radiotelephone communications energy received from a coverage sector. Processed radiotelephone signal selecting means is responsive to the processing means, for selecting at least two but less than all of the at least three processed radiotelephone communications signals. Decoding means is responsive to the signal selecting means, for decoding the radiotelephone communications from the at least two but less than all of the at least three processed radiotelephone communications signals.
The present invention offers the advantages of diversity reception of signals from multiple antenna arrays without requiring the increased complexity and cost of combining of all of the signals received from multiple antennas in a single diversity combiner, by selecting particular processed radiotelephone communications signals for input into a lower-order diversity combiner. The present invention can thus provide for a greater variety of combinations of processed radiotelephone communications signals. The present invention can also provide for enhanced diversity reception by preventing injection of noise from signals having poor quality, such as low carrier-to-noise ratio, into the diversity combiner. In addition, the present invention may allow the use of existing lower-order diversity combiners with higher-order multiple antenna arrays.
According to the present invention, the radiotelephone signal selecting means preferably selects on a basis such as highest power or signal quality. Preferably, the radiotelephone signal selecting means selects only two of the processed radiotelephone communications signals, to be passed on to a conventional two-branch combiner. The decoding means preferably comprises equalizing means, such as an equalizer for performing a weighted combining of the selected processed cellular radiotelephone signals according to certain characteristics, such as carrier to noise ratio and signal power. It will be understood that the radiotelephone receive antenna array may comprise at least three antenna elements, such as individual horizontal or vertical dipole antennas. It will also be understood that the processing means may comprise beamforming means, such as beamforming hardware for producing antenna beams from an antenna array, with one or more beams covering a coverage sector.
In another aspect of the present invention, the processing means may include means for producing at least two processed first polarization radiotelephone communications signals, each representing cellular radiotelephone communications energy received from a coverage sector and having a first polarization and at least two processed second polarization radiotelephone communications signals, each representing radiotelephone communications received from a coverage sector and having a second polarization. The two polarizations preferably are orthogonal to each other, for example, horizontal and vertical.
The processed radiotelephone signal selecting means selects at least one of the at least two processed first polarization radiotelephone communications signals and at least one of the at least two processed second polarization radiotelephone communications signals. The selecting means preferably selects from the processed radiotelephone communications signals representing each polarization on a basis such as highest power or highest signal quality. The present invention thus provides for enhanced polarization diversity reception.
The cellular radiotelephone receive array may comprise at least three antenna array elements, such as dipole antennas. The four antenna array elements may comprise at least two antenna elements for receiving radiotelephone communications energy having the first polarization and at least two antenna elements for receiving radiotelephone communications energy having the second polarization.
The cellular radiotelephone signal processing means may comprise beamforming means, such as beamforming hardware for producing antenna beams from an array of antenna elements. Preferably, each beam covers a coverage sector and receives cellular radiotelephone communications energy having a particular polarization. Preferably, the coverage sector of an antenna beam having the first polarization will overlap with the coverage sector of an adjacent antenna beam having the second polarization.
A method aspect of the present invention operates a cellular radiotelephone base station for communicating with at least one cellular radiotelephone. Cellular radiotelephone communications energy, representing cellular radiotelephone communications from cellular radiotelephones, is received on a cellular radiotelephone receive antenna array. The received cellular radiotelephone communications energy is processed to produce at least three processed radiotelephone communications signals, each representing the cellular radiotelephone communications energy received from a coverage sector. At least two but less than all of the at least three processed radiotelephone communications signals are selected. The radiotelephone communications from the at least two but less than all of the at least three processed radiotelephone communications signals are decoded. Enhanced cellular radiotelephone communications are thereby provided, without the need to add undue complexity to the base station elements.